Galvanic anode



United States Patent 3,274,085 GALVANIC ANODE Herbert C. Rutemiller andAllen M. Montgomery, Cleveland, Ghio, assignors to Aluminum Company ofAmerica, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing.Filed Apr. 25, 1963, Ser. No. 275,497 7 Claims. (Cl. 204-148) Thisinvention relates to aluminous metal galvanic anodes for the cathodicprotection of metals and relates particularly to consumable anodes of analuminum base alloy for the cathodic protection of non-aluminous metalstructures exposed to the corrosive action of aqueous media andparticularly aqueous saline media.

Cathodic protection systems are well known in which a metal articleimmersed in an electrolyte is protected from corrosion by means of asacrificial or consumable anode which is also immersed in theelectrolyte and is electrically connected to the metal structure(cathode) which is to be protected. Protection against corrosion isparticularly important when the metal article is exposed to thecorrosive action of aqueous saline media. Sacrificial anodes areemployed to provide cathodic protection for such structures as steelpipe lines, ship hulls, ship ballast tanks, meta-1 sea walls, anddrilling rigs.

Sacrificial or consumable anodes are generally made in any desired shapeor size to suit the structure to be protected and must be composed of ametal which is anodic to the metal body to be protected. The anodes maybe in wrought or cast form but the latter has generally been preferred.Some convenient means for at taching the anode to the article to beprotected is usually necessary such as an embedded metal core strap, rodor cable.

For many applications the expense of replacing consumed anodesrepresents a substantial part of the cost of the protective system. Forthis reason it has been recognized that a long life accompanied byadequate current output is highly desirable for reducing the cost ofcathodic protection. This characteristic is referred to as high currentefficiency and is generally expressed in terms of ampere hours ofcurrent delivered to the cathode per pound of anode metal consumed. Thedifference in potential between the anode and cathode must be greatenough during the life of the anode to maintain an adequate flow ofcurrent and to polarize the cathode to a sufficiently high potential toavoid local corrosion cells on the cathode.

It is also desirable that anodes have a high electronegative solutionpotential in open circuit, which is expressed in volts relative to somestandard electrode, without sacrificing current efiiciency. An aluminumbase alloy anode having a higher or more electronegative potential thananother aluminum base alloy of the same size will protect a largercathode area than the aluminum base alloy anode with lesselectronegative potential.

Two very desirable features of an aluminum base alloy anode, therefore,are high current efficiency and high electronegativc potential in opencircuit.

An object of this invention, therefore, is to provide an aluminum basealloy galvanic anode having a high current efficiency and capable ofmaintaining a high electronegative solution potential.

Another object is to provide an improved aluminum base alloy galvanicanode which has a more electronegative solution potential thanheretofore attainable in an anode having substantially the same highcurrent efficiency.

We have found that a very small amount of boron in aluminum-zinc-tinalloy type anodes which have been solution heat treated in accordancewith the disclosure in copending patent application Serial No. 143,042,filed October 5, 1961, now US. Patent No. 3,227,644, of which one of usis the inventor, produces an anode having a more electronegativesolution potential than the same anode without boron. The presence ofboron, it has been discovered, increases the electronegative potentialof the anodes without any significant change in their currentefiiciency. From a practical standpoint this means that these anodesprotect a larger cathode area than the same anode without boron. This,of course, is an important economical factor to be considered whendesigning cathodic protection systems. The alloy used for such anodesshould consist essentially of aluminum, from 3.5% to 9.0% by weight ofzinc, from 0.05% to 0.20% by weight of tin and from 0.02% to 0.0 8%boron. To obtain the best results, we prefer to use from 6.0% to 8.0% byweigh-t of zinc. All impurities in the aluminum base alloy, such as forexample iron, silicon and copper, should not exceed a total of 0.50% andmore specifically the alloy should contain less than 0.20% iron, 0.20%silicon and 0.02% copper since in greater amounts they reduce thecurrent efficiency of the treated anodes. All other impurities shouldnot be over 0.05% each.

The zinc component of the alloy is necessary to provide the desiredbasic electrode potential for the heat treated anode. Smaller amountsthan 3.5% do not supply the desired characteristics in the anode whilemore than 9.0% does not produce any added improvement in performance.The element tin also favorably affects the behavior of the treatedanodes. It has been found than tin serves to maintain a high level ofsolution potential over the life of the anode. Smaller amounts than thestated minimum do not produce a significant increase upon the solutionpotential whereas larger quantities produce no added improvement andincreases the cost of production. With respect to boron, less than 0.02%does not produce a significant increase in solution potential while morethan 0.08% does not produce any additional increase.

As described in copending application Serial No. 143,042, referred tohereinabove, the solution heat treatment of the alloy required toestablish the desired condi tion for high anode performance consists ofheating the anodes to a temperature between 800 F. and 925 F., andholding within this range for a sufi'lcient length of time to effectsubstantially complete solution of the soluble alloying elements andthereby establish a homogeneous internal structure. Generally the periodof soaking within the foregoing temperature range should extend over aperiod of from 1 to 12 hours, the length of time being dependent uponthe temperature and mass of the anodes being treated. Holding the anodeswithin the aforementioned temperature for about 2 hours has been foundin many instances to be sufficient and can be considered to be apractical minimum for commercial heat treatment. Heating to atemperature in the lower portion of the temperature range usuallyrequires a longer time to bring about a solution of the soluble elementsthan heating within the upper portion of the temperature range. Once thealloying elements are in substantially complete solution and ahomogeneous condition is created there does not appear to be anyadvantage to continue the thermal treatment.

After the anodes have been held at the elevated temperature for asufficient length of time, they should be rapidly cooled to roomtemperature. This can be accomplished in a known manner as by quenchingin an air blast, by water spray, by immersion in a water bath, or byother means. The particular cooling means employed will in general bedetermined by the facilities at hand. In order to reduce warpage of theanodes, we

3 have found a quench in hot water at a temperature of about 180 to 212F. to be quite satisfactory. No further thermal treatment is necessaryor desirable after the drastic cooling operation.

The anodes can be made in either cast or wrought form but generally itis most convenient to produce them in the form of castings since thesupporting rod or cable can be cast in place. The sand or permanent moldcasting procedures are generally most convenient to employ. The size andshape of the anodes will vary with the type of installation, andgenerally weigh between and 50 pounds.

An open circuit potential difference of 0.2 to 0.4 volt between thetreated aluminum alloy anode and the steel structure insures adequateprotection on the one hand while on the other hand avoiding what isknown as over protection. The presence of boron does not produce overprotection, we have found, even though that element serves to raise thesolution potential.

The improvement in anode solution potential resulting from the additionof boron to the anode alloy and solution heat treated under the sameconditions is illustrated in the following example:

Three sample anodes were tested. Anode No. 1 was composed of an aluminumbase alloy without boron and consisting essentially of aluminum, 7.18%zinc and 0.12% tin, with an impurity content of 0.002% copper, 0.03%iron, and 0.05% silicon. Anode No. 2 was composed of an aluminum basealloy consisting essentially of aluminum, 7.32% zinc, 0.11% tin, and0.03% boron, with an impurity content of 0.002% copper, 0.03% iron, and0.05% silicon. Anode No. 3 was composed of an aluminum base alloyconsisting essentially of aluminum, 6.78% Zinc, 0.11% tin, and 0.07%boron, with an impurity content of 0.003% copper, 0.03% iron, and 0.05%silicon. All three of the anodes were given a solution heat treatmentconsisting of heating for 8 hours at 875 F. followed by a quench inboiling water before being tested. Each of the anodes were weighed andimmersed in synthetic sea water in separate steel drums whose interiorsurfaces had been sand blasted prior to the test to remove all rust andscale. The anodes were lectrically connected with the drums through a0.05 ohm resistance. The exposure period of the anodes was 21 days. Eachday during the test period the circuit was broken and theelectronegative potential of the anodes measured with a Leeds andNorthrup Speedomax recording potentiometer with respect to a 0.1 Ncalomel electrode. After the initial reading there was no significantdeviation in the readings for each of the anodes throughout the rest ofthe exposure period. As determined from the readings anode No. 1 had aninitial electronegative potential of 1.190 v. and a final potential of1.180 v., whereas under the same conditions anode N0. 2 had an initialelectronegative potential of -1.190 v. and a final potential of 1.240 v.and anode No. 3 had an initial electronegative potential of --1.190 v.and a final potential of 1.230 v.

At the end of the 21-day test period the anodes were 4 removed, cleanedand weighed to determine the loss of metal. The current efficiency ofboth anode No. 2 and No. 3 was at least equal to that of anode No. 1.

Having thus described our invention and certain embodiments thereof weclaim:

1. A thermally treated galvanic anode composed of an aluminum base alloyconsisting essentially of aluminum, from 3.5% to 9.0% of zinc, from0.05% to 0.20% tin, and from 0.02% to 0.08% boron, the alloy containinga total of not over 0.50% of all impurities, said anode having ahomogeneous internal structure resulting from a solution heat treatmentand rapid cooling to room temperature and characterized by 'a moreelectronegative solution potential than the same anode without boron.

2. A thermally treated galvanic anode in accordance with claim 1,wherein the amount of zinc is from 6.0% to 8.0%.

3. A thermally treated galvanic anode composed of an aluminum base alloyconsisting essentially of aluminum, 3.5% to 9.0% zinc, from 0.05% to0.20% tin, and from 0.02% to 0.08% boron, the alloy containing asimpurities up to 0.20% iron, up to 0.20% silicon, up to 0.02% copper andall others not exceeding 0.05% each, the total of all impurities notbeing over 0.50%, said anode having a homogeneous internal structureresulting from a solution heat treatment and rapid cooling to roomtemperature and characterized by a more electronegative solutionpotential than the same anode without boron.

4. A thermally treated galvanic anode according to claim 3 wherein thezinc content is 6.0% to 8.0%.

5. A thermally treated cast galvanic anode composed of an aluminum basealloy consisting essentially of aluminum, from 3.5% to 9.0% of zinc,0.05% to 0.20% tin, and from 0.02% to 0.08% boron, the alloy containinga total of not over 0.50% of all impurities, said anode having ahomogeneous internal structure resulting from a solution heat treatmentand being characterized by having a more electronegative solutionpotential than the same anode without boron.

6. A thermally treated cast galvanic anode in accordance with claim 5,wherein the amount of zinc is from 6.0% to 8.0%.

7. A thermally treated cast galvanic anode composed of an aluminum basealloy consisting of aluminum, from 3.5% to 9.0% zinc, 0.05% to 0.20%tin, and from 0.02% to 0.08% boron, the alloy containing as impuritiesup to 0.20% iron, up to 0.20% silicon, up to 0.02% copper and all othersnot exceeding 0.05% each, the total of all impurities not being over0.50%, said anode having a homogeneous internal structure resulting froma solution heat treatment and being characterized by having a moreelectronegative solution potential than the same anode without boron.

No references cited.

JOHN H. MACK, Primary Examiner.

T. TUNG, Examiner.

1. A THERMALLY TREATED GALVANIC ANODE COMPOSED OF AN ALUMINUM BASE ALLOYCONSISTING ESSENTIALLY OF ALUMINUM, FROM 3.5% TO 9.0% OF ZINC, FROM 0.5%TO 0.20% TIN, AND FROM 0.02% TO 0.08% BORON, THE ALLOY CONTAINING ATOTAL OF NOT OVER 0.50% OF ALL IMPURITIES, SAID ANODE HAVING AHOMOGENEOUS INTERNAL STRUCTURE RESULTING FROM A SOLUTION HEAT TREATMENTAND RAPID COOLING TO ROOM TEMPERATURE AND CHARACTERIZED BY A MOREELECTRONEGATIVE SOLUTION POTENTIAL THAN THE SAME ANODE WITHOUT BORON.